Identifying Infectious Diseases · 3 min read
The Chain of Infection and Pathophysiology
The chain of infection describes the six links that must be present for an infection to spread. Every infection prevention intervention works by breaking at least one link.
The Six Links
| Mode | How it spreads | Healthcare examples |
|---|---|---|
| Direct contact | Physical transfer between an infected or colonized person and a susceptible host | MRSA on hands, scabies |
| Indirect contact | Transfer through a contaminated intermediate object (fomite) or hands | C. difficile on bed rails, shared equipment |
| Droplet | Respiratory droplets travel a short distance (generally within about 3 to 6 feet) and land on mucous membranes | Influenza, pertussis, N. meningitidis |
| Airborne | Small particles (droplet nuclei) remain suspended and travel on air currents over long distances | Tuberculosis, measles, varicella |
| Common vehicle | A single contaminated item exposes many people | Contaminated medication vials, food, water |
| Vector-borne | Insects or arthropods carry the pathogen | Malaria, West Nile virus (rare in healthcare) |
Stages of Infectious Disease
The incubation period runs from pathogen entry to symptom onset and ranges from hours (toxin-mediated food poisoning) to years (some viral infections). Incubation periods are essential for outbreak investigation, for defining exposure windows, and for setting quarantine or work-exclusion periods. Patients can be infectious during the prodromal period, before the diagnosis is obvious, and some remain infectious during convalescence.
Colonization, Infection, and Healthcare-Associated Infection
Colonization means organisms are living on a body site without invading tissue or causing symptoms; colonized patients are important reservoirs. Common sites include the anterior nares (S. aureus), the gastrointestinal tract (VRE, ESBL-producing and carbapenem-resistant organisms), and the skin. Infection occurs when organisms invade tissue and provoke a host response; it may be local or systemic. Bacteremia is bacteria in the bloodstream; sepsis is a dysregulated host response to infection that causes organ dysfunction.
A healthcare-associated infection (HAI) is acquired in the course of care. For NHSN surveillance in acute care, an infection is healthcare-associated when its date of event falls on or after hospital day 3, counting the day of admission as day 1; an event on day 1 or 2 is present on admission. Major HAI types include central line-associated bloodstream infection (CLABSI), catheter-associated urinary tract infection (CAUTI), surgical site infection (SSI), ventilator-associated events, and C. difficile infection.
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